The present invention pertains to removal of moisture or water vapor from gaseous hydrogen chloride (HCl).
Gaseous hydrogen chloride is one of the key specialty gases used in the manufacture of semi-conductor devices. HCl is used to grow silicon crystals (epitaxy) and is also used to etch silicon wafers in a dry etching process. For both of these applications high purity HCl is required. High purity HCl is generally taken to mean HCl free of water, or containing less than 0.5 ppm of water.
HCl containing water is highly corrosive and will present problems to the gas delivery system as well as adversely affecting production yields during the manufacture of semi-conductor devices.
For example water contained in hydrogen chloride gas will necessitate frequent replacement of piping, manifolds, valves and the like which are used to deliver the hydrogen chloride to the point of use in the semi-conductor manufacturing process. In cleaning susceptors which are the support structure on which wafers are processed, water contained in gaseous hydrogen chloride will result in a formation of a new oxide on the susceptor thus, frustrating the cleaning function of the hydrogen chloride. In etching applications, water contained in the hydrogen chloride becomes a source of undesirable contamination in a semi-conductor manufacturing environment, which may render chip products made in this environment less than efficient or totally useless for their intended purpose.
U.S. Pat. Nos. 4,853,148 and 4,925,646 disclose a number of adsorptive compositions and methods for removing water from HCl. Among the materials and processes suggested is adsorption using magnesium chloride (MgCl.sub.2) supported on an alumina (Al.sub.2 O.sub.3) substrate. However, it has been found that the use of MgCl.sub.2 on Al.sub.2 O.sub.3 for moisture removal from hydrogen chloride results in hydrogen chloride slowly reacting with the Al.sub.2 O.sub.3 support to produce a volatile product, AlCl.sub.3. The AlCl.sub.3 will solidify at cool spots in the downstream process equipment resulting in deposits on the process equipment, such as regulators, filters and valves. This in turn causes system maintenance problems and eventually contamination of the HCl product.
Japanese Patent Application H5-13695 discloses removal of water from gaseous hydrogen chloride using a synthetic mordenite. However, it was found that the synthetic mordenite is not stable in gaseous hydrogen chloride.
Russian Patent 1726368 discloses removing water and oxygen from hydrogen chloride gas using iron chloride (FeCl.sub.2) supported on a carbon substrate.
Prior art workers have also used silica gel to remove water from gaseous hydrogen chloride. However, although silica gel and FeCl.sub.2 on carbon are stable in hydrogen chloride their water adsorption capacities are relatively low so that these materials must be changed more frequently when used in a HCl purification system.